In the manufacture of a spectacle lens, cutting and polishing of the lens surface to be machined of a cast and molded blank (semi-finished spectacle lens) form an optical surface having a desired curved surface. The general shape of the spectacle lens forms a meniscus, and its optical surface comprises a convex curved surface and concave curved surface.
Examples of the optical surface shape of the spectacle lens comprise, in addition to a spherical surface, an aspherical surface, a progressive surface, an aspherical surface having a progressive element, and the like to correct various types of refraction errors of an eye ball. Usually, the lens surface to be machined of the spectacle lens is cut by a lathe or milling machine, as disclosed in, e.g., European Patent Laid-Open No. EP0849038A2.
The machining path of cutting usually forms a helical shape. Cutting is done by setting the cutter to have a constant feed pitch in a horizontal direction perpendicular to the helix of the cutter. Accordingly, the helix spacings of the helical machining path become equidistant on an X-Y projection plane, as disclosed in, e.g., Japanese Patent Laid-Open No. 2003-525760, and do not become constant on the lens surface but decrease gradually from the outer periphery of the lens toward the center. This is due to the following reason. As the machining spindle exists on an X-Y plane, setting the feed pitch in the horizontal direction to have constant spacings can facilitate control. As the number of types of lenses is large, it is cumbersome to calculate helical machining paths that match the individual lens shapes, and control them. In the present invention, the X-Y projection plane refers to a plane in, e.g., a meniscus spectacle lens, which is perpendicular to the optical axis of the lens.
As a prior art, for example, Japanese Patent Publication No. 7-67659 proposes a machining data generating method which minimizes the cutting resistance of a cutter on the basis of a plurality of patches (surface elements that form a geometric model curved surface) on a free-form surface.